The inclusion of carbon fibre in the manufacturing of wind turbine blades is quite new and although its use is fairly extensive amongst the most important companies in the sector, the experience in relation to the method used for protection against lightning is still fairly limited and there is no general knowledge of any technique whose efficacy has been demonstrated.
Both the regulations applicable to wind turbines and existing literature on the subject, speak of an efficient method of protecting fibre glass blades against lightning consisting of a lightning protection system based on occasional receptors located along the length of the blade and a cable which connects said receptors to the blade root and passes through its interior.
In this sense the following patent can be quoted WO 96/07825 which presents a “Lightning protection for wind turbine blades” comprising an electrical conductor which extends from the tip of the blade travelling through the inside of the blade and ending at the blade root in a pin rod which allows the turning of the blade. The conductor will be with or without insulated covering and contributes to the support of the blade itself.
From WO 0177527 a blade is known which presents a protection against lightning with internal and external conductors, connected to each other and distributed along the whole blade. It also presents a series of penetration points and a pair of configurations for the tip and the rest of the blade.
U.S. Pat. No. 6,612,810 presents a protection whereby the blade comprises a pair of conductors extended lengthways along the surface of the blade (it also incorporates anti-ice heating elements). The blade tip has an impact receptor connected to a third conductor which runs through the inside of the blade. All conductors and heaters are connected to each other.
Existing literature on the impacts of lightning also state the need for equipotentialising the object which will receive the impact with the lightning protection system. In other words, in order for all elements to be at the same potential they must be electrically connected by conductors located on the area to be protected.
Carbon fibre, as a conducting material, must be kept equipotential with the lightning protection system. The problem with leaving conductor elements insulated is the high difference in potential which is created between them due to induction phenomena caused by the lightning when it passes through the lightning protection system. This difference in potential may give rise to arcing, which could be fatal in the case concerning us where the carbon fibre laminate constitutes the main resistant part of the blade.
There are different metal mesh based systems which attempt to prevent any intervention of the carbon fibre compound in the potential differences problem yet none of them have been shown to be efficient at solving the problem.